Add-on circuit boards are becoming larger and more complex, and generate more heat. These boards are commonly mounted vertically with a massive on-board heat sink and cooling fins forming an assembly to remove the heat into the ambient air flow. Various vertical mounting brackets have been employed to provide lateral support for the added weight of the sink and fins. Horizontally mounted circuit board assembles are secured to the mother board and do not require the vertical lateral support. However the horizontal boards do not tower above the mother board, where they can be cooled on both sides by ambient air. U.S. Pat. No. 5,889,656 issued on Mar. 30, 1999 to the present inventor (currently allowed application Ser. No. 08/862,656 filed on May 23, 1997, assigned to the present assignee) discloses a vertical mother board bracket with pivoting retaining arms for receiving an add-on circuit board. The pivoting permits three positions of the retaining arms with respect to the mother board, a folded storage position, an open receiving position, and an unfolded retaining position. In the folded storage position, the retaining arms are folded generally horizontally close to the mother board in a low height storage profile. In the open receiving position the retaining arms are pivoted beyond the vertical of the unfolded position for permitting ready installation of the add-on circuit board assembly. In the unfolded retaining position the retaining arms are unfolded generally vertically for mechanically retaining the ends of the add-on circuit board assembly after installation.